Methods for Preparing Selectively-Releasable Adhesives

ABSTRACT

In one embodiment, a method for preparing a selectively releasable adhesive includes condensing a multifunctional alcohol and a multifunctional carboxylic acid to form a prepolymer and curing the prepolymer at an elevated temperature and a vacuum to produce a cured polymer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to copending U.S. non-provisionalapplication entitled “Selectively-Releasable Adhesives” having Ser. No.12/171,739, filed Jul. 11, 2008, which is entirely incorporated hereinby reference.

BACKGROUND

Adhesives are used in many applications, including consumer, industrial,and medical applications. Although some adhesives are intended to form apermanent or semi-permanent bond with the items to which they areapplied, many adhesives are used in less permanent applications. Thelatter type of adhesives are often used in applications in which anarticle is to be affixed to something else and later removed. An exampleis the common adhesive bandage strip, which is intended to stick to theskin until the wearer wishes to remove the bandage strip.

A problem with the adhesives that are used in temporary applications isthat the adhesive may still adhere well to an object to which it hasbeen applied when the time for removal has arrived. For example, in thecase of an adhesive bandage strip, such adhesion can make it moredifficult to remove the bandage strip and therefore may cause discomfortto the wearer. Although such discomfort may be relatively mild, thediscomfort from removal of other types of bandages can be much greater.For example, removal of medical tape that secures dressings to the skinof a burn patient can not only cause the patient a great deal of pain,but further cause tissue damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed adhesives can be better understood with reference to thefollowing drawings. The components in the drawings are not necessarilyto scale.

FIG. 1 is a schematic illustration of the chemical structure of aselectively-releasable adhesive.

FIG. 2 is a diagram that illustrates an example chemical structure foran embodiment of the adhesive of FIG. 1.

FIG. 3 is a graph that plots peel strength of PGS tape as a function ofcure time.

FIG. 4 is a graph that plots peel strength of PGS tape after applicationof various solvents.

FIG. 5A is a diagram depicting adhesion between the adhesive of FIG. 2and human skin.

FIG. 5B is a diagram depicting reduced adhesion between the adhesive ofFIG. 2 and human skin after the application of a solvent.

FIG. 6 is graph that plots peel strength of PGS tape both before andafter application of an ethanol solution as a function of time.

FIG. 7 is a perspective view of an embodiment of adhesive tape thatincorporates a selectively-releasable adhesive.

FIG. 8 is a side view of the adhesive tape of FIG. 7.

FIG. 9 is a side view of an embodiment of an adhesive bandage strip thatincorporates a selectively-releasable adhesive.

FIG. 10 is a front view of a patient to which adhesive ECG leads thatincorporate a selectively-releasable adhesive have been applied.

FIG. 11 is a top view of an embodiment of an ECG lead shown in FIG. 10.

DETAILED DESCRIPTION

As described above, it can be difficult to remove articles that havebeen affixed to an object using conventional adhesives. Moreover, incases in which the article is a bandage and the object is a patient,pain and/or tissue damage can occur from such removal. Described in thefollowing are adhesives that lose much of their adhesive strength when asolvent is applied. Therefore, such adhesives can be used inapplications in which selective release of the adhesive or an article towhich the adhesive is applied is desired. In some embodiments, theadhesive comprises poly(glycerol-sebacate) and the solvent comprises analcohol. Example applications for the adhesives include use in glues andadhesive tapes or bandages.

In the following, various embodiments of adhesives and articles thatincorporate adhesives are described. Although specific embodiments arepresented, those embodiments are mere exemplary implementations and,therefore, other embodiments are possible. All such embodiments areintended to fall within the scope of this disclosure.

Adhesive Compound Synthesis and Characteristics

Referring now to the figures, in which like reference numerals identifycorresponding features, FIG. 1 schematically illustrates an adhesivecompound or polymer 10, referred to herein simply as “adhesive.” Asindicated in that figure, the adhesive 10 includes a polymer backbone 12and a plurality of chemical bonds including hydrogen bonds 14 andcovalent bonds 16. As is apparent from FIG. 1, there are a relativelylarge number of hydrogen bonds 14 as compared to covalent bonds 16.

In some embodiments, the adhesive 10 comprises a copolymer formed from amultifunctional alcohol and a multifunctional carboxylic acid. As usedherein, the term “multifunctional alcohol” refers to any alcohol thathas two or more hydroxyl (—OH) groups, and the term “multifunctionalcarboxylic acid” refers to any carboxylic acid that has two or more acid(—COOH) groups. Example multifunctional alcohols include glycerol,monomeric carbohydrates such as glucose and mannose, and small polyolssuch as oligo (vinyl alcohol). Example multifunctional carboxylic acidsinclude diacids such as sebacic acid, succinic acid, oxylic acid, andmalic acid, and triacids such as citric acid. One example of such acopolymer is poly(glycerol-sebacate) or “PGS.” An example of synthesisof PGS is described in the following several paragraphs.

The sebacic acid used to prepare the PGS can be rigorously purifiedprior to synthesis of the PGS to improve the quality of the resultantpolymer. Such purification can be performed by combining a relativelysmall amount of sebacic acid with a relatively large amount of ethanoland heating the mixture until the sebacic acid completely dissolves.Once the sebacic acid has dissolved, the hot sebacic acid solution canbe filtered under a vacuum and the filtrate can be refrigerated forseveral hours to enable crystallization. The sebacic acid crystals arethen collected and intermittently filtered under vacuum to collect thecrystals. After the completion of the filtration, the above process(dissolution, crystallization, and filtration) can be repeated multipletimes (e.g., 3-4 times) to ensure a high level of purification.Thereafter, the air-dried sebacic acid crystals can be heated under avacuum to remove any residual ethanol or moisture.

Once the sebacic acid has been purified, it can be used to synthesizePGS. Such synthesis can be accomplished through melt polycondensation ofan approximately 0.5:1.0 to 1.5:1.0 molar ratio of glycerol to purifiedsebacic acid at an elevated temperature, such as approximately 120° C.In some embodiments, an equimolar amount (i.e., a 1.0:1.0 ratio) ofglycerol and sebacic acid may be used. The reaction can, for example, becarried out under nitrogen gas (N₂) flow. The mixture can be stirred andwater distilled from the reaction can be trapped and removed. Thecompound that results is a prepolymer of glycerol and sebacic acid,i.e., PGS prepolymer. As used herein, the term “prepolymer” describesthe polymer prior to curing. Accordingly, the prepolymer exhibits nocrosslinking.

After the PGS prepolymer has been synthesized, it can be transformedinto the PGS polymer using a curing process. In some embodiments, theprepolymer is first mixed with solvent to form a solution that can besprayed on a suitable non-stick substrate. Once the solvent evaporates,a film of PGS prepolymer remains on the substrate that can be cured inan oven over a period of several hours. FIG. 2 illustrates the chemicalstructure of an embodiment of a resultant PGS polymer 18, and thehydrogen bonding that occurs between chains of the polymer.

The duration of time over which the prepolymer is cured (i.e., the curetime) directly affects the physical characteristics of the resultantpolymer, including its adhesive strength. This is apparent from thegraph of FIG. 3. As indicated in that graph, maximum adhesive strengthwas achieved for a preparation of PGS synthesized in the mannerdescribed above after approximately 9 hours of curing at approximately120° C. and 100 milliTorr (mTorr). Specifically, testing of PGS adhesivetape (i.e., a substrate comprising a layer PGS) in accordance with ASTMD3330-78 PSTC-1 (using a reduced peel speed of 3.8 millimeters persecond (mm/s)) revealed that the greatest peel strength occurred whenthe PGS adhesive was cured for approximately 9 hours at approximately120° C. and 100 mTorr.

Unexpectedly, the strength of the adhesive, as exhibited by the peelstrength, increases as the adhesive is cured, at least until the 9 hourmark, suggesting that the strongest adhesive is obtained when some, butrelatively little, crosslinking occurs. This may be due to the fact thatcrosslinking increases the cohesiveness of the adhesive, which reducesseparation of the adhesive from itself and, to a point, increasesadhesive strength. In view of this, a cure time at or near 9 hours at atemperature of approximately 120° C. and a pressure of approximately 100mTorr may be preferable for some applications. Notably, that cure timeis significantly smaller and that vacuum is significantly weaker thanthat used to form other PGS polymers, such as the “bio-rubber” disclosedin U.S. Patent Publication Number 2003/0118692 of Wang et al. In thatpatent application, a non-adhesive elastomer is described as beingformed by curing a PGS prepolymer for 48 hours at 40 mTorr and 120° C.

In addition to affecting adhesive strength, the shorter curing time andweaker vacuum also affect the chemical structure of the resultingpolymer. For example, when the PGS prepolymer is cured for a relativelyshort period of time, such 20 hours or less, the resulting PGS polymerhas a relatively large number of hydrogen bonds, which are particularlybeneficial to obtaining good adhesion of human skin. Furthermore, asmentioned above, cure time affects the degree of crosslinking thatresults.

The crosslink density of PGS polymer was quantified using two differentapproaches. In a first “equilibrium swelling” approach, PGS samples weremixed with tetrahydofuran (THF). The samples were placed into sealedvials to minimize solvent evaporation. Each day, the excess THF on thesurfaces of the samples was removed and the samples were weighed togauge the extent of the swelling until no significant weight increasewas observed, typically after about 3 days. At the end of that timeperiod, the samples were blotted using filter paper and again weighed todetermine for each sample the mass of the swollen network atequilibrium, meq. The samples were then dried in a vacuum oven at 25° C.for 1 week to determine the mass of the dried network after extractionof the solvent, md. Swelling was then calculated as a swellingpercentage using the following relation:

$\begin{matrix}\frac{m_{eq} - m_{d}}{m_{d}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

The crosslink density was then calculated using the Flory-Rehnerexpression for tetra-functional affine network as expressed by:

$\begin{matrix}{{\upsilon = \frac{{\ln ( {1 - \upsilon_{2}} )} + {\upsilon \; 2} + {\chi \; v_{2}^{2}}}{\upsilon_{1}( {( {\upsilon_{2}/2} ) - \upsilon_{2}^{1/3}} )}}{and}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack \\{\upsilon_{2} = \lbrack {1 + {( \frac{m_{eq} - m_{d}}{m_{d}} )( \frac{\rho_{2}}{\rho_{1}} )}} \rbrack^{- 1}} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack\end{matrix}$

where ν is the strand density, ν₁ is the molar volume of the solvent, ν₂is the volume fraction of the polymer at equilibrium swelling, X is thepolymer-solvent interaction parameter (i.e., the Flory-Hugginsparameter, X=0.42), ρ₁ is the density of the polymer (i.e., 1.15 g/cm³),and p₂ is the density of the solvent (i.e., 0.889 g/cm³). Assuming anideal tetra-functional network, the crosslink density, n, is half thestrand density, ν.

Using the above approach, the crosslink densities of the samples after7, 9, and 16 hours were respectively determined. The results of thosedeterminations are provided in Table 1.

TABLE 1 Cure Time υ₁ υ n n (mol/L) (h) Swelling Percentage υ₂ (mol/L)(mol/L) (mol/L) (mean ± std) 7 (6.1404-0.5013)/ 0.103 0.0811 0.037 0.0180.5013 = 11.25 7 (5.6754-0.4152)/ 0.093 0.0811 0.030 0.015 0.4152 =12.67 7 (6.2596-0.4352)/ 0.088 0.0811 0.027 0.014 0.0157 ± 0.21% 0.4352= 13.38 9 (5.4115-0.4608)/ 0.108 0.0811 0.041 0.021 0.4608 = 10.74 9(4.538-0.3879)/ 0.108 0.0811 0.041 0.021 0.3879 = 10.69 9 (6.1-0.5424)/0.112 0.0811 0.044 0.022 0.0213 ± 0.01% 0.5424 = 10.25 16(5.5778-0.6226)/ 0.140 0.0811 0.071 0.036 0.6226 = 7.96 16(5.0197-0.5471)/ 0.137 0.0811 0.068 0.032 0.5471 = 8.18 16(6.3256-0.6786)/ 0.135 0.0811 0.066 0.033 0.0337 ± 0.21% 0.6786 = 8.32

In a second approach, crosslink density was derived from Young's modulusmeasured during mechanical testing. In this approach, samples of PGSwere immersed in water to identify their volumes and briefly reducetheir tackiness. The samples were then individually reshaped into solidcylinders and mounted between glass cover slips and mounted between thecompression platens on a Synergies 100 mechanical tester equipped with a50 Newton (N) load cell. Tests were performed according to ASTM D695with a pre-load force of 0.1 N, a pre-load speed of 1 millimeter perminute (mm/min), a strain endpoint of 0.4 mm/min, and a test speed of 1mm/min. The cross-sectional area was calculated for each sample bydividing the sample volume by the specimen height as determined by thecrosshead distance minus the thickness of the two glass cover slips. Themolecular mass between the crosslinks, M_(c), was then calculated usingthe following equation:

$\begin{matrix}{M_{c} = \frac{E_{o}}{3{{RT} \cdot \rho}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

where E_(o) is the Young's modulus of the sample as determined from themechanical testing, R is the universal gas constant, T is thetemperature in Kelvin, and ρ is the density of the sample. The crosslinkdensity derived from modulus, n_(E), for each of the samples was thendetermined using the following equation:

$\begin{matrix}{n_{E} = \frac{( {\frac{M_{w}}{M_{c}} - 1} )}{( {\frac{M_{w}}{M_{r}} - 1} )}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

where M_(w) is the molecular weight of the PGS prepolymer as measuredusing gel permeation chromatography and M_(r) is the molecular mass ofthe repeating unit as determined by the known characteristics of theconstituents of the material (i.e., glycerol and sebacic acid).

Using the second approach, the crosslink density percentages after 7, 9,16, and 20 hours were respectively determined. The results of thosedeterminations are provided in Table 2.

TABLE 2 Cure Time (h) n_(E) (%) 7 0 9 1.24 16 2.76 20 7.00

In view of the above, PGS adhesives having desirable adhesive strengthcan be obtained by curing the polymer for up to approximately 20 hoursat approximately 120° C. and 100 mTorr. As is apparent from FIG. 3, curetimes in the range of approximately 6 to 12 hours yield polymers havingthe greatest adhesive strength, with the maximum adhesive strength beingexhibited for cure times of approximately 8 to 10 hours (e.g., 9 hours).Adhesives cured within those ranges of time may possess crosslinkdensities of approximately 0 to 0.5 moles per liter (mol/L) and/orapproximately 0 to 4 percent. Maximum adhesive strength may be achievedwith adhesives having crosslink densities of approximately 0.01 to 0.04mol/L and/or approximately 1 to 3 percent. While particular cure timeshave been identified, it is noted that cure time is dependent upon otherparameters, such as temperature, pressure, and sample size. Therefore,equivalent degrees of curing may be achieved in greater or lesser timedepending upon those other parameters when curing is performed.

As mentioned above, the adhesive strength of the disclosed adhesive canbe selectively reduced through the application of a solvent. Suitablesolvents include alcohols, such as ethanol, 1-propanol, 2-propanol, and1-butanol; ketones, such as acetone, and methyl ethyl ketone; ethers,such as tetrahydrofuran and diethyl ether; amides, such as N,N-dimethylforamide; sulfoxides, such as dimethyl sulfoxide; and esters, such asethyl acetate. The effectiveness of ethanol, 2-propanol, acetone, andmethyl ethyl ketone are exhibited in the graph of FIG. 4. The resultsfrom that graph were obtained using the same ASTM procedure identifiedabove 10 seconds after the various solvents were sprayed onto PGS tapesamples. As can be appreciated from FIG. 4, a 70/30 solution of ethanoland water and a 70/30 solution of 2-propanol and water both reduce thepeel strength of PGS tape by more than 95%, while acetone and methylethyl ketone reduce the peel strength by more than 98%. Therefore,application of an appropriate solvent dramatically reduces theadhesiveness of the adhesive so that it possesses very little adhesivestrength. Notably, the loss in adhesive strength occurs rapidlytypically example within a few seconds. As a consequence ofcharacteristics, the adhesive or an article to which the adhesive hasbeen applied, such as medical tape or a bandage strip, can be removedwith very little force, and therefore very little discomfort and/ortissue damage, once the solvent has been applied.

A mechanism with which release of the adhesive may be achieved isdepicted in FIGS. 5A and 5B. FIG. 5A illustrates bonding of the adhesive18 of FIG. 2 to skin 20 prior to application of a solvent. As indicatedin FIG. 5A, hydrogen bonding interactions occur between the adhesive 18and the skin 20. FIG. 5B illustrates the adhesive 18 and the skin 20after the introduction of ethanol. As indicated in FIG. 5B, ethanolmolecules 22 swell the adhesive 18 and form hydrogen bonds with both theadhesive and the skin 20, thereby interfering with bonding of theadhesive to the skin. In addition, the swelling of the adhesive 18reduces the cohesiveness of the polymer and results in easier removal ofthe adhesive from the skin.

In at least some embodiments, the adhesive strength of theselectively-releasable adhesive is recoverable. Specifically, theoriginal adhesive strength of the adhesive returns after the appliedsolvent evaporates. Such results are shown in the graph of FIG. 6, whichillustrates peel strength of PGS tape both before application of a 70%ethanol solution (i.e., time=0) and multiple times thereafter. As can beappreciated from FIG. 6, although peel strength is dramatically reduced5 minutes after application of the ethanol solution, the peel strengthnearly returns to initial levels after 30 minutes.

In some embodiments, the adhesiveness of the selectively-releasableadhesive naturally degrades over time while exposed to moisture. Suchdegradation can be avoided or reduced by either storing the adhesive ina moisture-free environment, such as in a vacuum or in a water-free gas.For example, testing has shown that when PGS tape is stored in a weakvacuum of approximately 12 Torr or stored in N₂, the tape exhibited noreduction in peel strength for at least 60 days. Therefore, it may bedesirable to store articles to which the selectively-releasable adhesiveis applied in sealed packages under vacuum and/or that contain an inertgas until the time of use. Once exposed, the adhesive maintains much ofits adhesive strength for at least a week.

Example of Adhesive Synthesis

Samples of PGS adhesive were synthesized in a laboratory by adding 100grams (g) of sebacic acid to a 2 L round bottom flask along with 1 L of95/5 blend of ethanol and water. The flask was heated in a 55° C. waterbath until the sebacic acid completely dissolved. Once the sebacic aciddissolved, the hot sebacic acid solution was filtered through a 0.45micron (μm) nylon filter under a vacuum.

The filtrate was then transferred to a clean 2 L Erlenmeyer flask,allowed to cool to room temperature, and then stored at 4° C. overnight(approximately 8 hours) to enable crystallization. The next day, sebacicacid crystals were collected with a 0.22 μm filtration setup under avacuum. The crystals were maintained under the vacuum and intermittentlystirred for 3 to 4 days until the ethanol evaporated.

The above process was then repeated multiple times and the air-driedsebacic acid crystals were transferred to a 2 L glass beaker that wasplaced in a vacuum oven. A vacuum was applied until the oven reached afull vacuum of 90 to 100 mTorr, and then the oven was set to temperatureto 60° C., which was arrived at in 1° C./min steps. The sebacic acidcrystals were then maintained at 60° C. for a period of 16 hours.

Once the sebacic acid was purified in the manner described above, it wasused to synthesize the PGS by melt polycondensation. Equimolar amountsof glycerol (34.45 g) and purified sebacic acid (75 g) were heated toapproximately 120° C. in a 500 milliliter (ml) three-neck flask equippedwith a Dean-Stark trap under N₂ flow. The mixture was stirred atatmospheric pressure with a 1¼ inch×⅝ inch, egg-shaped stir bar having aweight of 15.5 g at stir rate of approximately 500 revolutions perminute (rpm). Water was collected in the trap during the stirring. Onceno more water collected in the trap, a vacuum was gradually applied overapproximately one hour until the pressure stabilized at approximately100 mTorr to 150 mTorr. The reaction was permitted to progress until themaximum stir rate was reduced to approximately 10 rpm (afterapproximately 72-90 hours). At that point, the mixture was a PGSprepolymer.

The PGS prepolymer was then was dissolved in THF to form a 30% PGSsolution and the solution was sprayed onto wax paper. The THF waspermitted to evaporate until only a film of PGS prepolymer remained. Thefilm was covered with a piece of cloth, which was transferred to avacuum oven. The cloth was then cured at 120° C. at 100 mTorr for 9hours.

Example Applications

The selectively-releasable adhesive described in the foregoing can beused in various applications, including consumer, industrial, andmedical applications. Described in the following are examples of suchapplications.

FIG. 7 illustrates an embodiment of adhesive tape 30 that incorporatesthe selectively-releasable adhesive. By way of example, the tape 30 canbe used in medical applications to secure bandages or other articles toa patient and therefore adheres to the patient's skin. The tape 30generally comprises a continuous, thin, and flexible strip having anouter side 32 and an inner side 34. FIG. 8 illustrates an exampleconstruction that can be used to form the tape 30. In the embodiment ofFIG. 8, the tape 30 includes a substrate 36 having an inner surface 38to which has been applied an adhesive layer 40 that comprises theabove-described selectively-releasable adhesive. In some embodiments,the substrate 36 comprises a flexible material that is adapted toconform to the contours of subjects to which the tape 30 is applied.Example constructions for the substrate 36 include layers of paper,textiles, polymers, foam, and foil. Irrespective of the material used,the substrate 36 preferably is porous so that a solvent applied to theexterior of the tape 30 can reach the adhesive layer 40 to facilitaterelease. By way of example, the adhesive layer 40 is approximately 10 to200 μm thick.

FIG. 9 illustrates an adhesive bandage strip 42 that incorporates theselectively-releasable adhesive. As indicated in FIG. 9, the bandagestrip 42 includes a substrate 44 having an inner surface 46 to which hasbeen applied an adhesive layer 48 that comprises the above-describedselectively-releasable adhesive. As with the substrate 36, the substrate44 can comprise a flexible material that is adapted to conform to thecontours of subjects to which the bandage strip 42 is applied. Exampleconstructions for the substrate 44 include layers of paper, textile,polymers, foam, and foil. Irrespective of the material used, thesubstrate 44 is preferably porous so that a solvent applied to theexterior of the bandage strip 42 can reach the adhesive layer 48 tofacilitate release. By way of example, the adhesive layer 48 isapproximately 10 to 200 μm thick. As is further indicated in FIG. 9, thebandage strip 42 includes a central dressing element 50 designed tooverlie a cut or other wound.

There are various other applications for the selectively-releasableadhesive beyond adhesive tape and bandages. FIG. 10 illustrates oneexample of such an application. Specifically, illustrated in FIG. 10 aremultiple adhesive electrocardiogram (ECG) leads 60 that have beenapplied to a patient 62. As indicated in the figure, wires 64 extendfrom electrodes 66 provided on the leads 60 to an ECG machine 68. FIG.11 illustrates an example configuration for one of the ECG leads 60. Asindicated in FIG. 11, the ECG lead 60 comprise a substrate 70 to whichis applied an adhesive layer 72 that comprises theselectively-releasable adhesive.

As stated above, the present disclosure describes various embodiments ofadhesives and articles that incorporate an adhesive. It is reiteratedthat those embodiments are mere exemplary implementations. Accordingly,although PGS adhesive has been described in detail, adhesives consistentwith this disclosure may be composed of other materials.

1-19. (canceled)
 20. A method for preparing a selectively releasableadhesive, the method comprising: condensing a multifunctional alcoholand a multifunctional carboxylic acid to form a prepolymer; and curingthe prepolymer at an elevated temperature and a vacuum for a time periodnot exceeding 20 hours to produce a cured polymer.
 21. The method ofclaim 20, wherein condensing comprises condensing glycerol and a diacid.22. The method of claim 20, wherein condensing comprises condensingglycerol and sebacic acid.
 23. The method of claim 20, whereincondensing comprises condensing an approximately 0.5:1.0 to 1.5:1.0molar ratio of glycerol and sebacic acid.
 24. The method of claim 20,wherein condensing comprises condensing an approximately equimolar ratioof glycerol and sebacic acid.
 25. The method of claim 20, wherein curingcomprises curing the prepolymer at approximately 120° C. and 100milliTorr.
 26. The method of claim 20, wherein curing comprises curingthe prepolymer the equivalent of approximately 8 to 10 hours atapproximately 120° C. and 100 milliTorr.
 27. The method of claim 20,wherein curing comprises curing the prepolymer the equivalent ofapproximately 9 hours at approximately 120° C. and 100 milliTorr. 28.The method of claim 20, further comprising purifying the multifunctionalcarboxylic acid prior to combining it with the multifunctional alcohol.29. The method of claim 28, wherein purifying comprises repeatedlydissolving and filtering the multifunctional carboxylic acid. 30-40.(canceled)